The following invention relates to a degaussing network system for demagnetizing a color CRT's steel aperture grill which may become magnetized due to changes in the position of the CRT relative to the earth's magnetic field and the like.
Color CRT displays frequently include degaussing networks for ridding the metal components of the CRT of magnetization caused by the earth's magnetic field or spurious fields produced by motors and other devices. Typical degaussing circuits of this type are shown in Frantz el al., U.S. Pat. No. 4,535,270 and in Truskalo, U.S. Pat. No. 4,636,911. The Frantz et al. '270 patent discloses a degaussing circuit which comprises a storage capacitor which stores a charge and applies that charge to a degaussing coil as a switch is turned on, permitting the coil and the capacitor to oscillate. The rapidly decaying oscillation of this circuit takes magnetized components of the CRT through a series of hysteresis loops and effectively reduces the magnetization of these components to very low levels. The Trusklo '911 patent is an improvement of the Frantz et al. device, and provides a second inductance connected in parallel with the degaussing coil to improve the "Q" of the oscillating circuit so that more oscillations can occur within the decay period of the oscillator network.
The Frantz and Trusklo devices time the occurrence of the degaussing operation relative to the CRT's vertical deflection pulses. This requires a rather complicated circuit arrangement to ensure that the degaussing current decays substantially to zero during a time when the vertical deflection current is substantially zero in order that no stray magnetic fields are created. Once degaussing is complete in both of these devices, however, a high voltage from the horizontal deflection power supply remains on the storage capacitors to await the next degaussing sequence. This high voltage remains on these parts as long as the CRT is turned on. This maintains a large, and potentially dangerous, potential during non-use periods, which is most of the time, and places considerable voltage stress on these parts of the circuit.